1. Field of the Invention
The present invention relates to optical modulators and drivers used in high speed optical transmission systems and more particularly to a hybridly integrated driver and external modulator device designed to have low voltage and power requirements.
2. Description of the Related Art
The expected growing demand in the telecommunications field for higher frequency broadband communications systems has spawned and sustained the significant efforts in the development of medium haul and long haul fiber high speed optical transmission systems. Some of the conventional limitations of such systems include optical fiber loss, fiber dispersion and optical noise. With the advent of optical amplifiers that amplify the optical signal without first converting it to an electrical signal, optical loss is ostensibly no longer the limiting factor. However, one of the most significant system limitations remains fiber dispersion. In particular, when high frequency laser light sources are modulated to carry a signal, they produce pulses having large wavelength shifts about their center transmission wavelength, called xe2x80x9cchirp.xe2x80x9d When applied to a dispersive medium such as optical fiber, the chirped pulses can become severely distorted when they reach a remote receiver, which can be tens or hundreds of kilometers away. Moreover, the severity of the dispersion problem increases with the spectral linewidth of the laser light source. In most modern medium and long haul systems that operate at a wavelength of 1.55 xcexcm, fiber dispersion effects can thus become significant.
The two types of modulating schemes typically used in optical transmission systems are direct modulation and external modulation. In direct modulation, the electrical signal is combined with a bias and applied directly to the laser source. In this way, the signal directly modulates the laser gain and thus the optical intensity output of the source. While simple to design, since conventional semiconductor laser direct modulation techniques result in large linewidth (due to frequency chirping), direct modulation is not usually employed in medium and long-haul links at presently-considered high bit rates (e.g. 10 Gbps and above). Rather, external modulation techniques that result in lower dispersion penalty due to the narrow linewidth of the sources are presently preferred.
Among external modulators, electro-absorption modulators (EAM""s) and electro-optic modulators (EOM""s) are used in high-speed links. Electro-absorption modulators are compact in size, require low driving voltages, have wide bandwidths and can potentially be integrated with lasers and drive electronics. Unfortunately, electro-absorption modulators also tend to result in chirping of the laser output (although not as severe as in direct modulation) and thus are typically only used for span lengths less than 80 Km at 10 Gbps and for even smaller spans at higher bit rates. For high-speed links with span distances greater than approximately 80 Km, traveling-wave electro-optic modulators are the modulators of choice. EOM""s can be phase modulators or intensity modulators, also called Mach-Zehnder modulators (MZM""s). EOM""s are fabricated in several material systems such as GaAs, InP, LiNbO3 (lithium niobate) and polymers, but LiNbO3 modulators are the most extensively used because they tend to have the best performance and reliability.
One of the major challenges in using conventional traveling-wave, electro-optic modulators is that they typically require high drive voltages. This problem is more severe at high bit rates because traveling-wave EOM""s have a bandwidth versus drive voltage tradeoff. Thus, high-speed EOM""s suffer from an even higher drive voltage requirement than other types. These higher drive voltages result in higher power dissipation in the modulator and driver circuits. Moreover, as bit rate requirements increase, the driver circuits that generate these high drive voltages to process these bit rates become increasingly difficult and expensive to manufacture.
Two important reasons for the high drive voltage and power requirements are: 1) non-idealities in the packages and interconnections between the traveling-wave modulator and the driver circuit; and 2) performance limitations in the driver and modulator due to the constraints of having standardized 50 xcexa9 interface impedances. It should be understood that the requirement of having 50 xcexa9 interface impedances stems from the use of industry standard 50 xcexa9 connectors, cables and test equipment. It is also understood that the standardized impedance constraint need not necessarily by 50 xcexa9Other standardized impedance constraints are possible, such as 75 xcexa9.
Several attempts at addressing the high-voltage problem at these high frequencies have been made. For example, in Noguchi et al., xe2x80x9cMillimeter-Wave Ti:LiNbPO3 Optical Modulators,xe2x80x9d Journal of Lightwave Technologyxe2x80x9d. Vol. 16, No. 4, April, 1998, the authors describe a Mach-Zehnder-type optical modulator with coplanar waveguide (CPW) electrodes, wherein a titanium-diffused waveguide is formed in a z-cut lithium niobate substrate in a ridge structure. As stated therein, an objective of the authors was to reduce conductor loss to lower the voltage requirement while maintaining velocity and 50 ohm impedance matching. While this design improved the performance of the modulator over other designs, it still included a 50 ohm impedance requirement as a design constraint.
The above-described drawbacks are present in the two common architectures for driving electrical signals in traveling-wave modulators, namely, single-drive and dual-drive modulation, which are now described in detail.
Single-drive traveling-wave electro-optic modulators have just one electrical signal input, as opposed to dual-drive modulators that have two electrical inputs with the electrical signals 180 degrees out of phase, and which are described next. Single-drive modulators are easier to use than dual-drive versions and are most commonly employed in commercial systems. Single-drive modulators can be fabricated from LiNbO3 (x-cut or z-cut), GaAs, InP or polymers. This discussion applies to single-drive modulators of all types, regardless of technology or fabrication methods.
FIG. 1 shows a conventional configuration for modulating an electrical signal onto an optical carrier using a single-drive modulator. In operation, an electrical input signal 12, which can be analog or digital, is first amplified with a packaged driver 30 and then modulated with a packaged modulator 20 onto an optical fiber 46 carrying light of a particular wavelength that is sourced by a continuous light source, such as a laser (not shown). Through the electro-optic effect in the modulator chip 22, the electrical signal is effectively converted into an optical signal that can be transmitted for long distances on the fiber 46 to an optical receiver.
The traveling-wave electro-optic modulator chip 22 and the driver chip 32 are enclosed in their own dedicated packages 20, 30, respectively. The interconnection between the driver circuit chip 32 and the traveling-wave modulator chip 22 is made through (a) a transition board 36 on the driver package 30; (b) a high frequency output connector 40 on the driver package 30; (c) an input connector 42 on the modulator package connected by a 50 xcexa9 coaxial cable 44; and (d) a transition board 24 on the modulator package 20. Each of the transition boards is connected to the chips via wire bonds and has planar transmission lines.
More particularly, the signal 12 is fed into the driver chip 32 of the driver package 30 from a first transition board 34 via wire bonds 38, and from the driver chip to a second transition board 36 via bond wires 39. The second transition board includes planar 50 xcexa9 transmission lines with input/output bond pads. The output of the transition board 36 goes to a 50 xcexa9 high-frequency output connector 40 that acts as the package feedthrough. The amplified signal is then routed over the 50 xcexa9 coaxial cable 44 to the input connector 42 of the modulator package 20. From the input connector on the modulator package to the input pads on the modulator chip, the signal must traverse over another transition board 24 similar to that in the driver package.
FIG. 2 depicts an electrical equivalent circuit for the system depicted in FIG. 1. The bond wires 28, 29 and 39 shown in FIG. 1, are shown here as sources of inductance 28xe2x80x2, 29xe2x80x2, and 39xe2x80x2, respectively, while the pads and discontinuities in the transmission lines act as reactive, generally capacitive, elements. Disadvantages to this conventional design include: (1) the bond wires and reactive discontinuities associated with every interface are a source of reflections for the electrical signals. Reflections need to be kept to a minimum since multiple bounces of the electrical signal are transferred to the optical signal in the modulator and can lead to intersymbol interference, jitter, eye closure, etc., which all cause degradation of the transmitted optical signal; (2) The transitions act as low pass filters and thus limit the bandwidth of the system. They also cause variations in the group delay. This bandwidth reduction slows down the rise and fall times of the pulses, while the variations in group delay can cause pulse spreading and overshoot, leading in turn to intersymbol interference and jitter; (3) The planar transmission lines on the transition boards, connectors and the coaxial cable introduce loss in the path of the electrical signal. Since a fixed drive voltage is required at the modulator input for acceptable system performance, high electrical loss in the transitions must be compensated by increasing the output voltage from the driver. Not only does this make the driver design problem hard due to breakdown issues, it also increases the power dissipation in the driver making thermal management harder; and (4) The cost associated with multiple high frequency packages, connectors and cables can be very high and often becomes significantly greater than the cost of the active components.
Another problem with the conventional scheme relates to the interconnection of the driver and modulator. In particular, the use of conventional 50 xcexa9 high frequency connectors and cables forces the driver output impedance and the modulator input impedance to be close to or at 50 xcexa9 in order to keep signal reflections to a minimum and thus power transfer at a maximum. Unfortunately, this 50 xcexa9 impedance constraint for the driver and modulator is a substantially design restriction for several reasons. First, as is well understood, traveling-wave EOM""s are designed to have velocity match between the optical mode velocity and the electrical signal velocity. The electrical signal velocity is a function of the electrode geometry as is the impedance of the electrodes. Specifying the modulator impedance to a conventional value, such as 50 xcexa9, places a constraint on the modulator""s electrode geometry and leaves little, if any, room for improving the electro-optic performance. This results in modulator""s having higher drive voltage requirements than might otherwise be required.
Moreover, the negative effects of forcing the driver output impedance to 50 xcexa9 are even more apparent. The driver output stage sees an effective impedance of approximately 25 xcexa9 because the driver output impedance appears in parallel with the modulator input impedance. This means that the driver needs to supply twice the current compared to a driver with infinite output impedance in order to generate the same output voltage swing. This results in higher power dissipation in the driver output stage.
Thus, it would be desirable to have this impedance constraint is lifted so that the electrode geometry could be designed for both velocity matching and optimum electro-optic performance, resulting in modulators requiring substantially lower drive voltages.
One of the most common configurations for using dual-drive modulators is the system 100 shown in FIG. 3, comprising a traveling-wave electro-optic modulator 122 and a driver circuit 132 enclosed in their own dedicated packages, 120 and 130 respectively. Two signal electrodes separated by a ground electrode each carry an equal, but 180 degrees out of phase, signal. Each signal independently controls the electro-optically induced phase shift in the two branches of the optical waveguide. Two main advantages that dual-drive modulators have been shown to have over single-drive modulators are adjustable chirp and reduced drive voltages. The advantages of variable chirp and the methods of achieving variable chirp using dual-drive modulators are well known in the art. Moreover, several groups, such as Namiki et al., have worked on reducing drive voltages through the use of dual-drive modulators. See for example, T. Namiki, et al., xe2x80x9cLow Drive Voltage Ti:LiNbO3 Mach-Zehnder Modulator Using A Coupled Linexe2x80x9d Tech. Digest, Optical Fiber Communications Conference, San Francisco, Calif. 1990, paper TUH4. Dual-drive modulators can be fabricated using LiNbO3, GaAs, InP or polymers and the comments made here apply to all dual-drive modulators.
The interconnection between the dual-drive driver circuit chip 132 and the dual-drive, traveling-wave modulator chip 122 is made through (1) transition board 136 on the packaged driver 130; (2) high frequency driver output connectors 140 and 141; (3) 50 xcexa9 coaxial cables 144 and 145; (4) high frequency modulator input connectors 142 and 143; and (5) transition boards 124 and 125 on the modulator package 120. Both the variable chirp and reduced drive voltage applications described above require the two signals arriving at the inputs of the modulator to be complementary, i.e. 180 degrees out of phase from each other. Thus, the transition boards, cables and connectors in the two signal paths must be phase matched so that the two signals remain 180 degrees out of phase at the modulator 122. While generating signals with 180 degrees of phase difference at the driver circuit output is relatively trivial, and, in fact, is readily obtainable with the use of differential pair stages, maintaining the phase difference all the way to the modulator chip 122 is very difficult when the signal must travel through all of these external cables and connectors. This task is further complicated by phase mismatching caused by temperature drifts.
Thus, it would be desirable to eliminate the need for phase-matched cables and connectors in dual-drive modulator systems. It should be understood that the disadvantages listed previously for single-drive packaged devices with 50 xcexa9 interface impedances also apply to dual-drive devices. The transition boards, connectors and cables cause signal loss and signal distortion. Moreover, the 50 xcexa9 interface impedance results in dual-drive modulators and drivers with non-optimal performance and thus higher voltage requirements.
Theoretically, the drive voltage required per signal electrode in the dual-drive case is half that required for the single-drive electrode and the power dissipated per signal electrode is 25% the power dissipated in the single electrode case. However, since two electrodes are to be driven, the net power dissipation for the dual-drive case is 50% that of the single-drive. This factor of two reduction in the drive voltage and power has very significant benefits, especially at very high bit rates, such as 40 Gbps, where it is very difficult to make driver circuits with large output voltage swings.
Thus, it would be highly desirable to have a high-speed traveling-wave modulator device having relatively low drive voltage and power requirements for a given bandwidth of interest.
The present invention, which addresses these needs, resides in an optical modulation device and method for modulating within a given bandwidth an electrical signal onto an optical carrier. The device includes an electrical driver that amplifies the signal to be optically modulated, including a driver circuit and an output impedance network, Zd, and a traveling-wave optical modulator. The modulator includes an electrical input connected to the driver, an optical waveguide linked to the optical carrier, and a traveling-wave electrode having an impedance, Zm. The modulator is configured to reduce the voltage required to drive the signal within the given bandwidth without conforming to a standardized impedance criteria, such as the 50 ohm standard of conventional modulators.
In one preferred embodiment, the driver and traveling-wave optical modulator are integrated in a single package and may more particularly be hybridly integrated in a single package. The modulator of the device of the present invention may be an electro-optic modulator. It may more particularly be a Mach-Zehnder electro-optic modulator. In one specific implementation of the device of the present invention, the electro-optic traveling-wave modulator is fabricated on a z-cut lithium niobate substrate and includes at least two electrodes geometrically configured to provide velocity match and a buffer layer disposed between the electrodes and the optical waveguide that is configured to increase the field overlap between the electrical signal and the optical waveguide, thereby reducing the drive voltage requirement of the modulator.
In another specific implementation of the optical modulation device that includes an electro-optic traveling-wave modulator, the modulator is fabricated on an x-cut lithium niobate substrate and includes at least two traveling-wave electrodes configured to provide velocity match and an electrode gap disposed between the electrodes that is configured to increase the field overlap between the electrical signal and the optical waveguide, thereby reducing the drive voltage requirement of the modulator.
In alternative embodiments, the modulator in the device of the present invention may be a phase modulator or an electro-absorption modulator, a single-drive modulator or a dual-drive modulator.
In any of the modulator embodiments of the device of present invention the driver impedance Zd may have a value that is chosen to reduce the power dissipation in the driver without conforming to a standardized impedance criteria, such as 50 ohms. In the preferred embodiment, Zd has a value that is chosen to minimize the power dissipation in the driver chip without conforming to a standardized impedance criteria.
In another implementation of the present invention, an optical modulation device for modulating within a given bandwidth an electrical signal onto an optical carrier is disclosed. This device includes an electrical driver that amplifies the signal to be optically modulated, including a driver circuit and an output impedance network, Zd and a traveling-wave, electro-optic optical modulator. The modulator has an electrical input connected to the driver, a traveling-wave electrode having an impedance, Zm, and an optical waveguide linked to the optical carrier. The modulator is configured to reduce the voltage required to drive the signal within the given bandwidth without conforming to a standardized impedance criteria, and the driver and modulator are integrated in a single package.
In yet another aspect of the invention, the optical modulation device of the present invention includes means for amplifying the signal to be optically modulated, and means for modulating the amplified signal onto the optical carrier, the means being configured to reduce the voltage required to drive the signal within the given bandwidth without conforming to a standardized impedance criteria. The device may further include means for integrating the means for amplifying and the means for modulating into a single package.
The present invention also resides in a method of reducing the voltage and power required to modulate within a given bandwidth an electrical signal onto an optical carrier. This method includes providing a driver that amplifies the signal, providing a traveling-wave optical modulator that includes an input connected to the driver, an optical waveguide and a traveling-wave electrode having an impedance, Zm, configuring the modulator to reduce the drive voltage needed to drive the signal within the given bandwidth without conforming to a standardized impedance matching criteria and combining the driver and modulator as a single device. In a more particular aspect of this inventive method, the step of providing the driver and modulator as a single device includes integrating the driver and modulator into a single package. Preferably, the driver and modulator are hybridly integrated in the package to improve performance.